Onyx is a low priced PFD with decent features. Their XL fits up to a 56" chest and the Torsion model has the bottom buckle, vee neck and the half back. It is also mesh ventilated for hot weather comfort and is only $80. Here is a link:
That said, I would strongly advise against paddling with the PFD unzipped. The buckle is meant to keep the vest from riding up and is generally not strong enough to prevent having the PFD ripped off your body if you are capsized in strong current. We lost someone a couple of years ago, a husband and father who was a volunteer on riverside trail cleanups – he was in his kayak near shore, wearing his PFD unzipped and leaned out to retrieve a trail sign that had fallen into the water. We had just had heavy storms the day before and the river current was fast and strong. He capsized and the force of the water swept him away, stripping off his PFD as he struggled, and he was sucked underwater and pinned beneath a barge parked along the shore, where he drowned before rescuers could get to him.
The Onyx vests are inexpensive because they don’t use as durable materials and hefty hardware as more pricey vests like Kokatat, Stohlquist, Astral and NRS. They are Coast Guard approved and will be safe if worn correctly, they just will not last as long as more costly PFDs with a lot of usage. The waist buckle on that Torsion certainly does not look like it would hold fast on its own on a large struggling person, even less so than with pricier vests. Again – it is not safe to paddle with any vest unzipped and not snugged up to fit the wearer closely.